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Hypoxia as related to human physiology describes a condition where there is an insufficient supply of oxygen to the body. Each of the billions of cells that make up the human body requires an adequate supply of oxygen for survival and optimum functioning. The supply of oxygen to all the cells is accomplished by the distribution of the oxygen in the air that we breathe-in being transferred into blood circulation through the alveolar (air-sac) epithelium of the lungs and carried by blood to all parts of the body. Although there are many situations where the body could become hypoxic, the cause that is relevant to aviation is the decrease in the amount of oxygen that is inhaled and transferred to the blood circulation, encountered by pilots when flying at high altitude. This essay discusses several aspects of the hazards of ‘hypoxia’ as it relates to aviation, its physiological effects on pilots, why it remains a threat even today and the available preventive measures to minimise the exposure to hypoxia.
High Altitude Atmosphere
Planet earth’s atmosphere is a mixture of gases comprising nitrogen (78%), oxygen (21%) with carbon dioxide and other rare gases making up the remainder. The layers of air in the earth’s atmosphere exert pressure, referred to as ‘atmospheric pressure,’ which understandably, is more on the earth’s surface and gradually lessen as we move away from the earth, deeper into the atmosphere, because the air gets lighter. The unit of measurement of atmospheric pressure is pounds per square inch (psi) or millimeters/inches of mercury (mmHg/inHg). Atmospheric pressure at sea level is 14.7 psi, or 760 mm/Hg.
In terms of human tolerance, three atmospheric zones have been recognized. They are the physiological zone at or closer to round level, the physiologically-deficient zone at high altitude and the space-equivalent zone (Alaska Air Medical Escort Training Manual). In the physiological zone that extend from sea level to 10,000 feet above sea level, despite the lowering of air pressure to about two-thirds of that at sea level, the oxygen partial pressure is adequate to support normal breathing in a healthy person without the need of oxygen supplementation.
The next zone that extends from 10,000 to 50,000 feet above sea level is quite risky to humans without breathing support. For example, at 50,000 ft altitude, the atmospheric
pressure is about 10% of that at sea level, with a corresponding decrease in oxygen partial pressure. This causes difficulties in inhaling enough oxygen as well as in the transfer of the relatively little oxygen in the lungs to the blood stream, leading to hypoxia. Above 50,000 feet altitude is what is called space-equivalent where humans cannot survive without the supply of an artificial environment.
The oxygen in the inhaled air, when carried to the air sacs in the lungs, gets to the bloodstream by a mechanism that is facilitated by the difference in oxygen partial pressure between the air sacs and the blood. The greater this pressure difference is, it is easier for oxygen molecules to transfer across the barrier. As the altitude increases the atmospheric pressure decreases, with a decrease in the partial pressure of oxygen. This makes the pressure difference between the air in the lungs and in the blood vessels (arteries) lower making it harder for the oxygen to transfer across the alveolar membrane and get into the bloodstream. For example, at sea level the alveolar partial pressure of oxygen (pO2) = 100mm, blood vessel pO2 = 40mm. Therefore, the gradient (difference) = 60mm. At 10,000ft altitude, the alveolar pO2 = 60mm and blood vessel pO2 = 26mm, with the gradient reduced to 34mm. It should be noted that the percentage of oxygen in the air at high altitude remains the same as at low altitudes but, the low partial pressure means that relatively fewer oxygen molecules are inhaled when breathing.
Physiological effects of hypoxia
Hypoxia is of special significance for those engaged in aviation (Cable, 2003). In general, aircrews do receive training in hypobaric chambers that provide similar conditions to 25,000 feet or greater altitude. They are trained to identify symptoms of hypoxia and instructions on how to respond. At high altitudes, the bodily reactions occur because the body attempts to compensate for the low oxygen supply. The immediate response is hyperventilation – an increase in the breathing rate as well as depth of each breath (gasping for breath) to increase the volume of air taken with each breath. There is an increase in heart rate, as the body tries to achieve enough oxygen supply to various vital organs. Many researchers have observed fluid collection in the lungs. This happens because the pressure in the blood vessels that surround the lungs increase causing effusion of fluid into the lungs (Cremona et al., 2002).
What is called altitude sickness is actually the physiological reaction by the human body to the environmental change from low altitude to above 8,000 feet (2,400 metres). It was a French physiologist Paul Bert who first demonstrated that this reaction is due to the deficiency in oxygen supply to the vital organs of the body (cited by Ghosh and Pant, 2010). The signs and symptoms of hypoxia include loss of peripheral vision, skin sensations (numbness, tingling, or hot and cold sensations), cyanosis (skin becoming bluish due to the accumulation of less oxygenated blood), euphoria, and eventually unconsciousness (Nishi, 2011). One of greatest dangers is the likelihood of the impairment of cognitive functions almost with the appearance of the first symptoms of hypoxia, resulting in possible inability to take any remedial action (Van Puyvelde et al., 2017).
The following case study reported by Ghosh and Pant (2010) illustrates the usual symptoms that a pilot exposed to hypoxia would experience. A test pilot of the Indian Air Force was scheduled to fly a high-altitude sortie. He took off in a modern combat aircraft, in suitable clothing; an Anti ‘G’ suit, Alpha Helmet and oxygen mask. He was flying at 46000 ft altitude (cabin altitude equivalent of 18,000 ft), when he felt a slight tingling on the left side of the upper lip. He responded by tightening the mask. Quickly the upper lip became numb. He switched the regulator to 100% Oxygen and then to overpressure position to deliver 100% oxygen at 2-3 mm Hg pressure. Still the symptoms didn’t go away, instead he also was developing a headache and a slightly dimmed vision. The pilot realized the danger and started immediate descent and reached 8000m altitude but, symptoms did not disappear. The sortie was aborted and descent to land initiated. The pilot recovered at about below 3000 m altitude.
Although hypoxia can occur at less than 10,000 feet altitude, it is generally not considered as a major problem for aviators (Department of Defence, 1998; Air Force Flights Standards Agency, 2006). In military helicopters without a pressurized cabin, pilots without oxygen supplement are permitted to ﬂy at/up to 10,000 ft (or at up to 13,000 ft for 2 hr) by the Japan Air Self-Defence Force (JASDF) (Japan Air Self Defence Forces, 2002). In the United States Air Force (USAF), pilots can fly continuously up to 10,000 ft, up to 12,500 ft for 1 hr, or 14,000 ft for 30 min, without supplemental oxygen. Nonetheless, hypoxic incidents have been reported in ﬂights even at below 10,000 ft. A report made to the Director of Flying Safety of the Australian Defence Force about incidents of hypoxia for the period 1990–2001, indicated that 4 out of a total of 27 incidents occurred at altitudes of less than 10,000 ft (Cable, 2003). Similar study by Smith (2005) reported that 40 of the 53 Australian Army helicopter pilots that were surveyed have experienced symptoms of hypoxia at less than 10,000ft altitude.
Schindler (2017) has drawn attention to the need to address hypoxia issues in relation to general aviators, considering that there are almost 600,000 certified general pilots (not engaged in military or commercial aviation and operate their own aircrafts for personal travel or recreation), in the US. Reporting an analysis of 130 hypoxic events reported by general aviators found the commonest symptoms experienced to be light-headedness (39%), headache (29%) dizziness (18%) and mental confusion (16%). Furthermore, the author drew special attention to the fact that most hypoxic incidents among general aviators go unreported and found that almost 70% of general aviators have had no hypoxic training.
In-flight hypoxia – Still a worry
The first recorded aviation victims of high-altitude hypoxia were two young French scientists Croce Spinelli and Sivel who in 1875 attempted to reach a 26,200 ft altitude in a balloon (Ghosh & Pant, 2010). Almost 150 years later. hypoxia continues to be major safety concern for aircrews who operate high altitude flights. Most recent incidents have been due to failure of pressure cabin or failure of the oxygen delivery system. The latter may be the result of inefficient ground servicing, mechanical failure of oxygen equipment, or an ill-fitting mask. Oxygen deficiency during flying not only causes fatalities but also reduces military capabilities. USAF data have shown that there were 10,700 reported incidents attributable to hypoxia during the second world war period 1941-45. Hypoxia incidents were responsible for 110 deaths and the abandoning of 1% of sorties, without accomplishing their missions (Gosh & Pant, 2010).
With the use of aircrafts that can fly faster and higher, preventing in-flight hypoxia has gained increased attention. The development of aircraft oxygen systems and their improvement have been very effective in addressing this concern. The first practical automatic oxygen delivery system was Dreyer’s apparatus designed by Col George Dreyer of British Royal Air Force (Gosh & Pant, 2010). These oxygen delivery systems became mandatory in 1919, for All US planes flying at high altitude. Research and development efforts since then helped in building the sophisticated aircraft oxygen systems that are currently in use. Despite the availability of such advanced and sophisticated systems, in-flight hypoxia still occurs, albeit less frequently.
Causes of Accidents
As mentioned earlier, most modern day in-flight hypoxia incidents are reported as due to failure of oxygen systems. The risk of accidents is increased because of the difficulties in recognising hypoxia symptoms for a person never exposed to it and the fact that more serious effects can happen so quickly leaving no time for corrective action. The subject would become unconsciousness and lose control of the aircraft. This is what was suspected to have happened in the October 1999 death of the world-famous golf champion – Payne Stewart, and four others. The crash of the Learjet they were flying was attributed to loss of cabin pressure resulting in the occupants becoming unconscious (The Guardian, 26th October 1999). Apparently, the jet flew out of control for 1,500 miles at 45,000 ft altitude and crashed on a rane of mountains in South Dakota, USA.
Cable (2003) analysed the hypoxia incidents between 1990 and 2001 in the Australian Defence Force reviewing all cases by aircraft type, number of occupants, number affected by hypoxia, deaths, symptoms experienced, and the altitude of incidence. Twenty-seven hypoxia incidents, involving 29 aircrew, had been reported. Unconsciousness occurred in only two one of whom died. Fighter and training jets with aircrew supplied with oxygen equipment were mostly (85%) involved, and mostly occurred between 10,000 and 19,000 ft. Mask or regulator failure including mask leaks accounted for most (63%) of incidents. Symptoms involved cognitive impairment or light-headedness as reported by aircrews. Aircrews were able to recognize the symptoms in majority (75.8%) of these episodes because they have received hypoxia training.
In another report (Gradwell, 2006), analysing 397 incidents of hypoxia reported to the British Air Force, found 57% of all cases were due to failure of oxygen regulator and cabin decompression. Other causes included breaks in the hose connecting the regulator and mask and face mask defects.Similar issues have been reported as the causes of oxygen supply problems leading to aircrews being exposed to dangers of hypoxia in the Australian study by Cable (2003).
There have been several incidents involving commercial flights too. The biggest disaster proven to be due to hypoxia-related incapacitation of the aircrew in a commercial flight is what happened to the Helios Airways Flight 522 on 14 August 2005 from Larnaca, Cyprus, The aircraft crashed into a mountain in Greece, 3 hours into the flight (Sgobba, 2014). The Air Traffic Control at Nicosia, after failing in repeated attempts to contact the aircraft, sent two F-16 fighter aircrafts. They did make visual contact with Flight 522 and reported that the first officer was slumped motionless at the controls while the captain was not at his seat. They also saw the freely dangling oxygen masks in the passenger cabin oxygen masks with no evidence of any movin passengers. Minutes later, both engines the plane crashed killing all 121 passengers and crew.
The disappearance, in March 2014, of the Malaysian Airlines Flight 370, a 777 carrying 239 people flying from Kuala Lumpur to Beijing, too is suspected as a hypoxia-related incident according to some experts. Evidently, the aircraft has changed course and travelled in southern direction, and experts think that it may have flown for up to seven hours on autopilot, run out of fuel and crashed into the Indian Ocean. They theorize that hypoxia caused by rapid decompression, just like what happened to Helios Flight 522 in Greece, must have been the reason (Sgobba, 2014).
Hypoxia from altitude exposure happens because of the lowering of oxygen partial pressure in the lungs to less than 60 mm Hg (Pilmanis, 2003). Therefore, civilian and military regulations make it mandatory for providing supplemental oxygen when flying above 10 000 feet of aircraft or cabin altitude. Sea level equivalent of oxygen can be attained by increasing the percentage of oxygen in the breathing gas, using an oxygen apparatus, up to an altitude of 34 000 feet. Positive pressure breathing with 100% oxygen is required above 40,000feet altitude (Pilmanis, 2003).
The development of aircraft cabin pressurisation system was a major breakthrough in efforts to safeguard against hypoxia (Pilmanis, 2003). Most modern aircrafts are pressurised. However, it has been reported that many civilian and military aircrafts do operate at times with no pressurising mechanisms. This is particularly common among general aviators who fly as high as 30 000 feet unpressurised. Oxygen equipment for hypoxia protection for such aircrafts range from simple nasal cannulas at lower altitudes to highly sophisticated regulators and masks at the higher levels. Large civilian and military passenger and cargo planes are equipped to maintain a cabin pressure that is equivalent to that of air at 4000–8000 feet altitude (Pilmanis, 2003). These large planes have a large cabin space and therefore, the effects of an accidental loss of cabin pressure develops slowly, so that there is plenty of time for the aircraft to descend to lower altitude. In contrast, in military aircrafts such as fighter jets and reconnaissance aircrafts cabin volumes are smaller and therefore, can lose cabin pressure very rapidly.
Following are some of the recommendations to reduce the incidents of in-flight hypoxia (Ghosh & Pant, 2010); The risk of hypoxia for aviators is real and therefore, this needs to be emphasized in all training programmes and manuals. Additionally, there is a need for the strict adherence to what is described as “oxygen system discipline” which includes taking a serious note of the dangers of hypoxia and knowing how to recognise and what to do in an incidence. All crew members must be aware of the potential for failure of oxygen delivery systems and therefore must be fully conversant with properly fitting of masks, importance of helmet mask compatibility, how to check and correct leaks, oxygen equipment checks, both pre-flight and in-flight. The importance and relevance of hypoxia training is evident from the findings that many pilots have averted disaster by being able to recognize symptoms early and take timely corrective action. It is equally important to ensure that each and every hypoxia incident is recorded, the information from which can be extremely useful in building strategies for continuous improvements in effective preventive strategy.
Considerable improvements have been made in aircraft pressurization systems and oxygen delivery systems, improving their performance and reliability enabling the protection of aircraft cabins to safeguard crews and passengers against hypoxia. However, the potential risk of hypoxia is still present as evident from recently reported incidents and unpublished reports that are confined to aviation agencies. The good news is that incidents are less frequent compared to the occurrences a few decades ago. Yet, there is no room for complacency and the safety of the pilots and those accompanying in an aircraft must be ensured always. The objectives of precautionary measures must be to meet the aviators’ physiological needs under all types of conditions like altitude, acceleration, temperature, and workload. The system testing must be done under simulated operational conditions across full range of anticipated use.
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